Acute whole body UVA irradiation combined with nitrate ingestion enhances time trial performance in trained cyclists
Introduction
Since Larsen and colleagues [1] first reported a reduced oxygen cost of exercise following ingestion of sodium nitrate, a growing number of studies have demonstrated the ergogenic effects of dietary nitrate supplementation on athletic performers [2], [3], [4], [5]. Research has shown that ingestion of nitrate-rich food such as beetroot and spinach can increase circulating levels of nitric oxide (NO) metabolites via an NO synthase (NOS) independent pathway [6]. Following absorption of nitrate from the stomach into the plasma, nitrate is transported into the saliva via the salivary glands. Bacteria then reduce the nitrate to nitrite [7] where, following ingestion, nitrite is potentially reduced to NO when exposed to hypoxic [8] or acidic environments [9].
This increase in NO related products, potentially leading to an increase in NO production, has been typically shown to reduce oxygen consumption (VO2) during sub-maximal steady-state exercise [1], [10] and improve both exercise capacity and performance [2], [3], [5], [10]. Conversely, several other studies report exercise performance of varying modalities to be unaltered by dietary nitrate supplementation [11], [12], [13]. While there are various methodological differences between studies that may account for this disparity, it has become apparent that ergogenic effects appear minimal in highly-trained endurance cohorts [11], [14]. This may be partly explained by the higher baseline nitrate/nitrite pool in endurance trained athletes compared to untrained matched controls [15]. Evidence from murine models also suggests that increases in muscle blood flow and contractile force production following nitrate supplementation only occur in Type II muscle fibers [16], [17]. One may, therefore, reasonably assume that elite endurance athletes, who are known to have high proportions of type I muscle fibers, would be less likely to benefit from nitrate supplementation [18]. Alternatively, using only a single low dose of dietary nitrate (~4–5 mmol) may explain the resultant diminished ergogenic effect during exercise [12], [13], [19].
Intriguingly, exposing the skin to the ultra violet (UV)-A component of sunlight increases circulating plasma nitrite via decomposition of photo reactive nitrogen oxides stored in dermal cells [20], [21]. Opländer and colleagues [21] observed that systemic blood pressure (BP) was reduced by 11% 30 min after UV-A exposure, a finding similar to that of dietary nitrate [22]. More recently, Liu and colleagues [23] provided some mechanistic basis for these findings by demonstrating UV-A induced NO production also increases forearm blood flow. Therefore, given that exposure to UV-A light increases NO bioavailability sufficiently to induce measureable physiological effects; it is plausible that it may also enhance exercise performance. Furthermore, it remains to be determined whether combining UV-A exposure with concomitant nitrate supplementation may potentiate a synergistic effect on NO bioavailability, given that both offer different routes for increasing plasma NO related products.
Despite this, the effects of UV-A exposure, either alone or in combination with nitrate supplementation, on plasma nitrite and parameters of exercise performance are currently unknown. This is of interest given the reported benefits to exercise performance associated with an increased availability of NO related products [2], [3], [5], [10]. Therefore, the aim of this study was to determine the effects of acute UV-A light exposure with and without simultaneous nitrate supplementation on plasma nitrite, the physiological responses to steady-state exercise and cycling time trial (TT) performance. We hypothesized that: (1) UV-A exposure would increase plasma nitrite and improve exercise performance and (2) UV-A exposure combined with nitrate would coalesce to increase plasma nitrite and improve exercise performance to a greater extent than either intervention alone.
Section snippets
Participants
Nine male trained-cyclists and triathletes (age 36 ± 6 years, stature 182 ± 5 cm, body mass 78.9 ± 6.0 kg, and VO2max 53.1 ± 4.4 mL⋅kg–1⋅min–1) volunteered and provided written informed consent to participate in the study that was approved by the School of Science Ethics Committee at The University of the West of Scotland. All participants were amateur competitive athletes who typically completed a minimum of two cycling training sessions per week and regularly competed in TT competitions. All
Plasma nitrite responses during intervention study
Plasma nitrite data from the intervention study is presented in Fig. 1. There was a significant main effect of ‘condition’ (P = 0.001) on plasma nitrite concentration. There was no significant main effect for time (P = 0.944) or a condition × time interaction (P = 0.083). Prior to the light-exposure, plasma nitrite in the NIT + SHAM (399 (345–461) nM) condition was higher than in the control (247 (179–343) nM, P = 0.024, 95% CI 17–257 nM). Plasma nitrite in the NIT + UV-A (391 (291–526) nM))
Discussion
Exogenous supplementation with dietary nitrate increases the bioavailability of NO which has been shown in some conditions to reduce the oxygen cost of exercise and improve performance [2], [3], [4], [5]. The present study explored the physiological and ergogenic effects of short-term exposure to UV-A light as a novel method to increase circulating NO metabolites both with and without ingestion of NIT. The principal finding was that exposure to UV-A light alone was not sufficient to
Conclusion
The principal findings of the present study were that exposure to UV-A light subsequent to ingestion of a NIT improved the physiological responses to steady-state exercise and 16.1 km cycling TT performance. Furthermore, we provide some evidence of a cumulative effect of dietary nitrate and UV-A derived NO, whereby the increase in plasma nitrite was larger than with either intervention alone. This study offers the intriguing possibility that a combination of naturally occurring environmental
Acknowledgments
The authors wish to thank Science in Sport who provided the NIT and PLA supplements for this study free of charge.
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